Typically, non-impact printing machines are designed to create a printed image by printing a series of picture elements, pixels or dots, on a print medium such as paper. In electrophotographic printing apparatus, the process of image generation is the same, although indirect, in the sense that the image is first formed on a photosensentive member, as a latent image, and then transferred to a paper sheet through a developing process. The image on then photosensitive member is formed by a light beam (usually a laser beam) which scans the electrically charged surface of the photosensitive material in a succession of scan lines. Each scan line is divided into pixel areas, and the light beam is modulated (e.g., switched on or off) at each pixel location, so as to selectively discharge the photosensitive material at the location of selected pixels. In this manner, an image is formed on the photosensitive member, the image consisting of pixels which have retained an electrical charge and pixels which have been discharged.
Thereafter, toner is applied to the surface of photosensitive material and adheres thereto in the locations (pixels) which have been discharged (in the case of positive or write-black process), or in the locations which have not been discharged (negative or write-white process). This step is known as the developing step and is followed in known manner by a transfer step (the toner is transferred to paper) and by a fixing step (the toner is fused onto the paper). The printed image produced by an electrophotographic printing apparatus its therefore a digitized image, or a bit mapped image, of a desired analog image which is represented and described at best by a discrete set of pixels or dots.
The resolution of the bit mapped images formed by electrophotographic printing machines is typically 300 dots per inch. No higher resolution is generally adopted because it would result in expensive and slow apparatus. With a resolution of 300 dots per inch, the discontinuities of the printed image as to a desired continuous image which has been digitized are readily perceivable, in particular when nearly vertical lines or edges or nearly horizontal lines or edges have to be printed. These lines or edges are effectively printed as stair steps showing undesired jaggies. Proposals have been made to overcome this limitation and to provide edge smoothing, without increasing the bit mapped image resolution, by using three basic approaches and combinations thereof:
1) Adding or substituting gray pixels for the normal black or white pixels at the edges of images;
2) Adding or substituting reduced size dots in pixels at the edges of images;
3) Adjusting the placement of dots as to the pixels in the scan direction.
In particular, U.S. Pat. No. 4,847,641 provides a summary of documents suggesting several approaches, and discloses a method and apparatus which uses a combination of approaches 2) and 3) above. All these approaches provide a partial solution to the problem of enhancing the quality of the image, but are not entirely satisfactory, particularly when lines or edges slightly slanted as to the scan direction of the light beam have to be printed.